Mechanical valves are employed in a wide variety of applications and used, frequently, to control fluid flowing through conduits such as pipe. One particular application where valves are employed involves the control of steam flow through steam turbines used in electric power generation. For example, Westinghouse Electric Company steam turbines employ several different types of valves, typically identified as throttle valves, governor valves, interceptor valves and re-heat valves. Similar valves are used on General Electric Company turbines. However, instead of the terms "throttle valve" and "governor valve", General Electric Company employs the terms "stop valve" and "control valve", respectively.
Presently, the extent to which some of these valves are opened or closed (position of the valve), is determined by a device known as a mechanical limit switch. A mechanical limit switch employs a centrally pivoting arm which attaches, at one end, to the end of a valve. The position of the valve end provides an indication of valve position and movement of the valve end applies a force to the end of the centrally pivoting arm to which the valve end is attached. That force causes the arm to pivot in relation to the position of the valve thereby swinging the opposite end of the arm about an arc. Positioned in proximity to that end of the arm are mechanical switches which are turned on or off when contacted by the end of the arm. Because the position of the arm is a function of the position of the valve, the position of the switches (on or off) is representative of the position of the valve. The switches, in turn, operate various indicator lights or provide signals to other electrical circuits. Because the position of the switches is representative of the position of the valve, such lights or signals, likewise, represent the position of the valve.
Such apparatus, however, is not without problems. A major problem occurs when the valve is changing position rapidly, which causes the pivotal arm to move rapidly which, in turn, causes the end of the arm to strike the electrical switches at high speed and force. That frequently causes such switches to fail, necessitating that the equipment be removed from service until repairs can be effectuated. Such switch failure also creates the risk that important safety equipment will not be properly signaled when the position of a valve changes, thereby defeating the built-in safety features and enhancing the possibility of an accident. Also, no dependable, readily available, highly accurate valve position indicators exist for interceptor and re-heat valves.
The present invention overcomes the inherent problems of such mechanical limit switches by eliminating the switches themselves, thereby eliminating a prime source of failure.